Much about the planet's future will depend on processes that humans today cannot directly observe - because they are occurring hundreds of miles below the sea surface where enormous marine glaciers, in Greenland and Antarctica, simultaneously touch the ocean and the seafloor.
The more we learn about this crucial yet inscrutable place, the more worrying it seems.
The latest exhibit: new research out of Greenland, conducted by Dartmouth earth sciences doctorate student Kristin Schild and two university colleagues, that has just been published in the Annals of Glaciology. The study examined the 3.4 mile-wide Rink Glacier of West Greenland, with particular focus on how meltwater on the ice sheet's surface actually finds its way underneath Rink, pours out in the key undersea area described above and speeds up the glacier's melt.
It's a feedback process that, if it plays out across many other similarly situated glaciers, could greatly worsen Greenland's overall ice loss. "These big tidewater outlet glaciers are the ones that are contributing these huge icebergs, they're the ones that have rapidly, rapidly sped up in the last decade," Schild said. This makes it critically important to learn "what are the main factors . . . that are leading to all these fast changes," she added.
Greenland is an enormous sheet of ice, capable of raising sea levels by some 20 feet if it were somehow to melt entirely and its waters were to pour into the ocean. Fortunately, it can't just do that all of a sudden - the vast ice sheet reaches the ocean only at relatively narrow, fingerlike glaciers that stretch out into fjords, or underwater canyons that lead out to the sea.
There are nearly 200 of these large outlet glaciers overall - and as Greenland goes, Rink is fairly large in size but far from the largest. It's less than 1 kilometer tall as it extends from the seafloor deep in a west Greenland fjord up above the surface of the water, Schild said.
That's hardly as massive as the nearby Jakobshavn Glacier, which has a base submerged well over a kilometer below sea level - and which is sending ice out into the ocean faster than any other in Greenland. But Rink, like Jakobshavn, touches the ocean across a wide, icy front, and is grounded deep below the surface of the fjord's waters. Here is where all the action is - including spectacular calving events, in which enormous icebergs break off, tumble into the water and eventually float out of the fjords.
There's growing concern that warming ocean waters are snaking into these fjords at depth and lapping at the glacier bases, making such breakups more likely. It doesn't help matters that scientists studiously mapping the fjords are finding, over and over again, that they're deeper than previously believed, creating more opportunities for the warm ocean to trigger melting.
But the situation is even more dynamic: Amid warmer atmospheric temperatures, Greenland is also melting on its surface, a process that forms vanishing lakes, ice-banked rivers and downward channels, called moulins, that carry meltwater deep beneath the ice sheet. This water then makes its way to the bases of outlet glaciers and, after traveling through complex passageways and, perhaps, being held up or stored in icy caverns, eventually flows out from beneath them and enters the sea.
It's the net consequence of all of these processes that will ultimately govern how quickly Greenland loses mass and causes the seas to rise. And that's what the new study gets at: It attempts to measure the mysterious process by which Greenland's surface meltwater eventually makes its way beneath the ice sheet and then out into fjords, by flowing to glacier fronts and escaping from underneath them.
To do so, the Dartmouth researchers used satellite imagery, as well as time lapse photography, to observe the seafront in the fjord where water touches Rink Glacier. They were searching for what they call "sediment plumes": When water rushes out from the glacier base and into the fjord, it's filled with sediments from the bedrock below. These pulses of water then ascend hundreds of meters to the surface and create an often colorful emergence there.
The study resulted in three separate new findings about how meltwater from Greenland's surface is making its way under Rink Glacier and speeding its ice loss - each of which suggests that not only Rink, but other glaciers like it, could lose their ice faster than previously thought.
First of all, the satellite and time-lapse images revealed that meltwater is pouring out from beneath Rink Glacier in not just one but four separate locations. That's bad news, because it means more overall melting of the glacier is possible. "Previously that has not been observed, to have more than one ocean location for a plume," Schild said.